This invention relates to microwave backdetector microcircuits and more particularly to a method of making a wideband microcircuit having a fragile backdiode therein. Tunnel diodes and backdiodes (a generic variation) are a special class of metal-semiconductor junction devices used in microwave circuits as detectors in wideband frequency band applications.
For many years it has been known that the operating characteristics of a tunnel diode can be precisely controlled by etching away the semiconductor material. A disclosure of such a method of forming a tunnel diode is shown in U.S. Pat. No. 3,150,021 to Akihiko Sato. As noted in that patent, a tunnel diode is mounted to a jig and immersed in an etching solution to etch the semiconductor material surrounding the connecting lead. Subsequently, the tunnel diode is removed from the jig member and is encapsulated for use in a circuit. As shown in FIG. 7 of U.S. Pat. No. 3,669,773 to Levi, a mesa type diode may be mounted on a conductive base. An insulating tube supporting a conductive ring surrounds the mesa diode. A gold ribbon is connected to the mesa diode and to the conductive ring. A conductive cap hermetically seals the mesa diode. This encapsulated diode can then be used in a suitable circuit.
Backdiodes have a very fragile neck. Conventionally, back diodes are packaged into a variety of "microwave packages," and are sealed to protect the fragile construction of the finished diode element. These packages are then incorporated into RF circuits and structures, as discrete components. These backdiodes, in the conventional encased or packaged form, present a number of performance limitations caused by degradation of the high frequency performance due to their residual and inherent parasitic reactances. In the prior art, there are frequency limitations that are severe in nature when attempts have been made to build a microwave limiter backdetector circuit or other microwave circuits utilizing a backdiode, because of the residual and inherent parasitic reactances associated with the required conventional package.
This invention provides, for the first time, a method of integrating backdiodes in RF and microwave circuits while advantageously drastically reducing or eliminating the troublesome residual reactances introduced by standard backdiode packages.
As an example of the frequency limitations in prior art backdetector circuits, it is noted that prior art limiter backdetector circuits are generally operable over a frequency coverage of not much more than one octave and are increasingly difficult to operate about 10 gigaHertz.